1. Field of the Invention
The present invention relates to a method of applying at least one layer of a releasable polymer coating on a glass substrate, especially a display glass and a hard disk blank, as a protective layer. The invention relates to electronic devices having display glass and to the display glass including the protective layer. The invention also relates to hard disk blanks made from glass or glass-ceramics.
2. Prior Art
The term xe2x80x9cglass substratexe2x80x9d, for the purposes of the present disclosure, means a substrate made from conventional glass or from glass-ceramics. There is a substantial yield problem in the manufacture of glass substrates, especially for display glasses and hard disk blanks, since the glass substrates have defects in the form of scratches and particles because of processing and transport. These scratches or cracks can also cause glass breakage, which is self-destructive when it occurs during processing, since the process must be interrupted in order to clean the machines of any splinters, which would otherwise cause further cracking or scratching.
Glass particles are released during cutting to length and breaking off glass pieces, which are deposited on the glass surfaces as abrasive particles and lead to scratches on the surface during further transport and working. Furthermore edge working after detaching the substrate leads to loading the glass surface with glass particles, which also act as abrasive bodies. Moreover a high pressure water stream is directed on the edges during edge processing to cool and to remove grinding solution.
Methods in which a polymer foil is laminated on both sides are known. These methods have several disadvantages. The protective foil can first be applied after cutting of the glass, which can thus be protected only during transport and edge working. During edge working the protective foil on the edges is partially lifted off the substrate and the grinding slurry collects there. As a result, the glass is damaged. Furthermore in thin glass particularly glass breakage increases because of the required drawing force for removal of the protective foil.
Adhesives are required for laminating the polymer protective layer on the substrate, which cannot always be removed without leaving a residue. The glass surfaces must also be subsequently polished, in order to obtain the required quality. The subsequent polishing is primarily mechanical and of course removes the adhesive residue, but does not produce a microscopically planar surface with optimum surface properties. This latter type of surface is desired especially for display glass and optical special glasses or also hard disk blank, since the polishing method produces minute cracks. The large amount of waste caused by glass breakage during polishing is a grave disadvantage.
A method is disclosed in German Published Patent Application DE 36 15 277 A1, in which flat glass is provided with a crack-resistant splinter-protecting coating. Immediately after manufacture, plastic powder is applied during cooling of the glass, which melts on the glass piece. A base layer and a crack-resistant covering layer are applied to increase the mechanical stability. The splinter-protecting layer made in this way may not be removed again. Furthermore the resulting surface does not have sufficiently good properties for use in displays. Furthermore it is not possible to use this product for hard disk blanks because of the plastic layer.
Japanese Published Patent Application JP-H10226537 describes a method, in which an aqueous protective film is applied, which does not react with the glass (and also contains no alkali salts) and is resistant to moisture. Polyvinyl alcohol solutions in water are used for the protective film. These polyvinyl alcohol solutions contain polyvinyl alcohols, which have an average molecular weight of at most 51,600 g/mol, preferably 25,800 g/mol, and which are partially hydrolyzed, preferably between 55% and 85%. The reasons for the upper limits are as follows: with higher average molecular weight and with hydrolysis degrees at about 95%, the water solubility greatly reduces. At small hydrolysis degree, in contrast, the water solubility increases with increasing hydrolysis degree. A protective film made from polyvinyl alcohol with an average molecular weight less than 50,000 g/mol and a hydrolysis degree between 55% and 85% already is very soluble in water at 30xc2x0 C. This sort of polyvinyl protective film is removed or dissolved in a very short time during edge working.
An additional method based on a water-soluble polymer-protective film is disclosed in the Patent BE 714,347. In this method a mixture of hydroxyethyl cellulose and dialdehyde in water or a mixture of polyvinyl alcohol and dialdehyde in water is prepared. The dialdeyde acts as a cross-linking agent, which makes the protective film capable of swelling and the adherence of the protective layer on glass is thus reduced. The water solubility of the polymer protective layer is adjusted by mixing the dialdehydes into it. No statement is made regarding the polymerization degree of the preferred polyvinyl alcohol. The hydrolysis degree should be at 85% or higher. It has been shown however that this protective layer is removed when the edges are worked.
It is an object of the present invention to provide a method, with which the surfaces of glass substrates are protected from destructive cracking and abrasive bodies during manufacture until the last stage of the processing, especially during wet edge working, and in all intervening transport steps, which has as few method steps as possible, which keeps waste to a minimum and guarantees as high as possible surface quality of the end product.
It is also an object of the present invention to provide a display glass, which has excellent surface quality with small manufacturing costs, and to provide an electronic device, which has display glass of this type.
It is another object of the present invention to provide a hard disk blank, which has excellent surface quality with reduced manufacturing costs, and applications for this type of hard disk blank.
According to the protective method of the invention, the glass substrates, especially display glasses or hard disk blanks, are coated with at least one removable protective layer of a polyvinyl alcohol with a molecular weight of greater than or equal to (i.e. not less than) 55,000 g/mol and a hydrolysis degree of greater than or equal to (i.e. not less than) 95% to form a removable protective layer. The at least one dissolvable or removable layer is only poorly soluble in cold water and withstands the wet edge working process. The glass substrate may be coated on only one side or on both sides as required.
The coating may be removed by washing with water at a temperature of 50xc2x0 C. or higher, especially 60xc2x0 C. and higher, preferably 70xc2x0 C. and higher, and most preferably 80xc2x0 C. and higher. The higher the water temperature, the faster the coating will be removed. The pH value is adjustable arbitrarily within the bounds of the glass resistance to acidic and basic conditions. The coating is rapidly removed in both acidic and basic media.
The original surface quality of the glass substrate surface is preserved by the at least one polyvinyl alcohol layer throughout the entire working process and during transport. The removal of the at least one polyvinyl alcohol layer by washing with water is, on the one hand, performed carefully, so that little glass breakage occurs, even with very thin glass, and on the other hand very thorough, since the at least one polymer layer is completely removed.
The surface quality of the glass would be impaired by microscopic cracks due to additional after-polishing and thus the waste due to broken glass would be increased. The after-polishing is therefore dispensed with for the glass made by the method according to the invention.
It has proven advantageous to employ a polyvinyl alcohol with an average molecular weight greater than or equal to 100,000 g/mol, especially preferably greater than or equal to 150,000 g/mol.
Similarly it as proven advantageous to use polyvinyl alcohol with a hydrolysis degree of greater than or equal to 97.5.
The water solubility of polyvinyl alcohol at room temperature with a high average molecular weight and very high hydrolysis degree is of course extremely poor (quasi-insoluble). In contrast, its water-solubility at high temperatures is very good. Because of that property this very highly polymerized polyvinyl alcohol is especially suitable to protect the glass substrate surface, even during the processing steps occurring in a wet edge treatment, in which the grinding solution is directed in a high pressure water stream for cooling and transporting at the edges.
Due to process engineering considerations it is advantageous to use an aqueous solution for application of the polyvinyl alcohol containing from 1 to 30% by weight, preferably from 1 to 20 percent by weight, and especially preferably from 5 to 10% by weight, of the polyvinyl alcohol.
In a second method according to the invention at least one layer of a polymer which is soluble in polar organic solvent is applied to the glass substrate. Preferably a polar organic solvent, such as an alcohol, ketone or ester, which is commercially available at a low price, is employed as the solvent in this second method. The polymer used for the coating is preferably a polyacrylate polymer, a polyvinylpyrrolidone/polyvinylacetate copolymer or polyvinylcaprolactam polymer.
The polymer coating is preferably again removed with an alcohol, a ketone or an ester and/or an aqueous solution of any of these organic solvents. Isopropanol or ethanol or aqueous solutions are especially preferred.
The original surface quality of the glass substrate surface is protected by the at least one polymer layer, also by the polyvinyl alcohol layer, during the entire working process and during transport. The at least one polymer layer is carefully removed by washing with organic solvents or aqueous solutions, so that little glass breakage occurs, even with very thin glass, but also very thoroughly since it must be completely removed.
Also in this second method the surface quality of the glass would be impaired by microscopic cracks due to additional after-polishing and thus the waste due to broken glass would be increased. The after-polishing is therefore dispensed with for the glass made by the method according to the invention.
In both methods the at least one polymer layer is preferably directly applied after making the glass substrates in order to preserve the surface quality attained during manufacture, before abrasive or dirt particles an reduce it. The application can occur directly at glass drawing.
The thickness of the at least one polymer layer is adjusted between 0.1 and 100 xcexcm according to the process steps to be performed and the application methods. A layer thickness of from 1 to 15 xcexcm has proven especially suitable and is preferred.
There are many possibilities for applying the polymer layer. Three preferred methods of application include dipping, rolling and spraying. The dipping is suitable for glasses, which are already pre-cut. The rolling is especially suitable for thick panels. The spraying, in contrast, is preferred for application of thin layers and especially for glasses, which should have the highest surface quality and thus should be still coated in glass drawing. Furthermore the spraying occurs without contact, so that contamination of the glass substrate surfaces is kept as low as possible. Preferably the high-volume low-pressure (HVLP) method is used, in order to produce as uniform a film as possible, and to keep the over spray, i.e. the excess spray fog or cloud to a minimum.
In as much as the glass piece or strip is not coated under warm conditions (e.g. 90xc2x0 C.), the drying process must be assisted. For that purpose either infrared or hot air can be employed. In both methods the glass is heated to a temperature of about 150xc2x0 C. The higher temperatures can lead to a modification of the protective layer or the glass. Lower temperatures, on the other hand, reduce the drying speed.
During infrared heating the glass is heated by radiation. Radiation in the wavelength range between 3 and 10 xcexcm has proven especially suitable, such as that from a glass black body radiator. Alternatively an electrically heated radiator can be employed.
During hot air drying hot air is directed on both sides of the glass strip or band. The dryer can also be constructed as a floating drier, so that the glass strip floats in the airflow and the drying occurs without contacting the glass strip or band.
If the coating should be removed in a shortest possible time, it is preferably to assist the washing by application of ultrasound waves. Thus the ultrasonic waves are propagated in the wash fluid and the coating is immediately loosened mechanically.
In a preferred embodiment a polymer layer, which is based on polyvinyl alcohol or a polymer which is soluble in polar organic solvent, is applied exactly on the glass substrate surface to be protected. Because of that the effort and expense of the process steps are reduced.
In special cases, in which the requirements for the protective layer are high, at least two polymer layers are preferably applied, which differ in their respective solubility, so that the layer applied to the glass substrate is more soluble than the layer applied over it.
In a preferred embodiment a first layer based on polyvinyl alcohol is applied to the glass substrate and a layer, preferably of a polyacrylate polymer, is applied over it. This has the advantage that the surface coating is very resistant to water and moisture, but also requires significantly less organic solvent for removal of the coating, than when equal thickness layers of polyacrylate polymer, or a similar water-insoluble polymer, are used. This provides an environmentally more friendly method, which stresses the environment less.
The invention also includes display glasses, which are not polished, and electronic devices, which are equipped with these display glasses. Furthermore the invention includes display glass, whose protective coating can be removed with polar organic solvent or water at temperatures greater than or equal to 50xc2x0 C. or aqueous solutions of polar organic solvents.
The invention also extends to hard disk blanks made from glass or glass-ceramics that are polished at no time point. Furthermore the invention includes hard disk blanks whose coating can be removed with polar organic solvents or water at temperatures greater than or equal to 50xc2x0 C. or aqueous solutions of polar organic solvents. The invention also includes hard disk blanks for use in electronic devices.